Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC. LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As the polishing pad, foamed polyurethane or the like having a pore structure having a function capable of retaining supplied slurry is used. However, such foamed polyurethane is poor in surface hardness. A polishing pad having hollow fine spheres entrapping high-pressure gas or water-soluble polymer powder dispersed in matrix resin such as polyurethane, as described in Japanese Patent No. 3013105 and JP-A 2000-71168, has been proposed as the polishing pad used in highly accurate polishing. Such a polishing pad secures a pore structure by the hollow fine spheres dispersed in polyurethane, and is endowed with high modulus of elasticity to a certain extent and rendered highly hard by the fine hollow spheres, thus improving flatness (relationship between the difference in level of a convex in a pattern in a device wafer and an abrasion loss of a concave in the pattern by polishing) to a certain extent. For example, IC-1000™ manufactured by Rodel is known as such a polishing pad. The hollow fine spheres can achieve high modulus of elasticity to a certain extent, but does not satisfy sufficient flatness because its effect of achieving high modulus of elasticity is limited due to the presence of the hollow structure. As is also described in a book, Toshiro Dohi et al.: Detailed Semiconductor CMP Techniques (in Japanese), Kogyo Chosakai, page 117 (2000), a hard pad capable of further improving flatness is necessary in consideration of application to elements in the next generation.
Further, a polishing pad having finely foamed or thermally foamed, fine hollow spheres dispersed in polyurethane is also described in JP-A 11-322877, JP-A 11-322878 etc. In particular, JP-A 11-322878 proposes a polishing pad having specific fine hollow spheres of finely foamed or thermally foamed polystyrene dispersed therein. However, such a polishing pad is not endowed with high modulus of elasticity and does not satisfy sufficient flatness.
Accordingly, an object of this invention is to provide a polyurethane composition capable of forming a polishing pad having a pore structure and being excellent in flatness.
For the purpose of improving flatness, a non-foamed hard polishing pad (for example, IC-2000™, manufactured by Rodel) can be used. As shown in the book supra (the same page supra), however, such a hard polishing pad causes the problem of scratches on the polished surface of a workpiece. The non-foamed polishing pad cannot sufficiently retain slurry abrasive grains on the surface of the polishing pad during polishing, and is thus not desirable from the viewpoint of polishing speed.
A polishing pad endowed with high elasticity by dispersing rigid solid beads in place of the hollow fine spheres is anticipated to improve the flatness of the polishing pad. Although flatness is improved by endowing the polishing pad with highly elasticity, the uniformity of a wafer as a whole is deteriorated when such a polishing pad is used. That is, both flatness and uniformity cannot be satisfied by merely dispersing rigid beads such as polystyrene beads. Further, the polishing pad endowed with high elasticity has a serious problem of scratches (scars) of the polished surface of a workpiece.
Use of polyurethane as the matrix resin forming the polishing pad is appreciated as described above. Such polyurethane is produced generally by curing an isocyanate-terminated prepolymer with a chain extender. A chlorine atom-containing chain extender such as 4,4′-methylenebis(o-chloroaniline), which is poor in reactivity and readily regulates curing reaction, is used as the chain extender. However, polyurethane (polishing pad) obtained by using the chlorine atom-containing chain extender as a starting material generates dangerous dioxins upon combustion after use and is thus not capable of thermal recycling (thermal recovery by combustion), thus raising a problem that there is no method for disposal except for landfill disposal.
Accordingly, an object of this invention is to provide a polyurethane composition capable of forming a polishing pad which can satisfy both flatness and uniformity and can reduce scratches. A still other object of this invention is to provide a polyurethane composition being capable of flatness improvement and scratch reduction which cannot be simultaneously achieved by the conventional polishing pad, being capable of thermal recycling of the polishing pad after use, and being not harmful to the environment.